Police in Bolivia confiscated around two tons of uranium found near the U.S. embassy and Spanish embassy in the capital, La Paz.

Deputy Interior Minister Jorge Perez told the EFE news agency that four Bolivians were arrested in connection with the uranium, which looked like chunks of sedimentary rock. The rocks was being stored in a building in cloth sacks.

“This material is used for building nuclear weapons, and was found in a van in the garage,” he told the El Dario publication.

He said that officials assumed that it was transferred into the country from another source because Bolivia does not produce uranium itself, according to EFE. Brazil and Chile are both uranium-producing countries, and Perez assumed the substance came from either of the two.

“For now, we can report that we presume the existence of uranium in these materials,” Carlos Romero, the minister of the presidency, told the Bol Press. He added that the substance still needs to be studied, and according to a preliminary study, the amount of actual uranium is minimal.

The preliminary laboratory test found that the material is “slightly radioactive” and “produces itchy skin” upon contact, according to the Bol Press. It is also insoluble in hydrochloric acid.

The seizure of the radioactive substance was the result of a six-week-long investigation, Eddy Torrez, the commander of a Bolivian elite police unit, told EFE.

Perez added that one of the suspects who was arrested is an engineer who told police that he held the uranium for others.